The invention relates generally to contact assemblies and in particular to a spring-biased, grid contact assembly and method of manufacturing thereof.
Integrated circuits are typically housed within a package that is designed to protect the integrated circuit from damage, provide adequate heat dissipation during operation, and provide electrical connection between the integrated circuit and the leads of a printed circuit board. Several conventional packages are in the prior art including land grid array (LGA), pin grid array (PGA), ball grid array (BGA), and column grid array (CGA).
In integrated circuit (IC) packages, terminal lands are arranged on one major face of the package in a pattern corresponding with mounting pads, or leads, on the surface of a circuit board or the like. The device package is mounted on the circuit board by soldering the terminal lands to the mounting pads. Packages having a pattern of lands distributed over a major portion of one face thereof are called land grid array (LGA) packages. Similarly, packages having small solder bumps arranged in a pattern on one face for forming interconnections with external circuitry are usually referred to as ball grid array (BGA) packages.
In many applications, the soldering of the leads of the IC package to the printed circuit board is undesirable. For example, it is impossible to visually locate a short or ground between the IC package and printed circuit board. Usually, an expensive X-ray technique is required to inspect the connections since the leads are hidden under the package. Further, the increasing number of leads being provided by IC packages makes the soldering of the packages to printed circuit boards more difficult.
Accordingly, in the prior art, an improved connector has been developed which is designed to eliminate the need for the soldering the leads of an IC package to a printed circuit board. One example of a device which satisfied this criteria is the wadded wire of “fuzz ball” socket. The “fuzz ball” socket comprises a non-conductive substrate formed with a plurality of through holes which each house a contact element. The contact elements are formed by forcing a predetermined length of gold plated wire into a through hole such that the wire will bend haphazardly into a jumbled contact that extends through the through hole and resembles a piece of steel wool. To mount an IC package to a printed circuit board, the “fuzz ball” socket is tightly sandwiched between the printed circuit board and the package to tightly secure to the “fuzz ball” socket. It can be appreciated, sufficient pressure must be applied to both the “fuzz ball” socket and the package, respectively, to maintain electrical connections between the lands of the package and the printed circuit board via the “fuzz ball” socket.
As the number of lands and corresponding “fuzz ball” contacts are increased, the pitch between contacts is maintained increasing the module size correspondingly with increased manufacturing problems due to the number of contacts. The placement of individual wires into evermore through holes requires tremendous logistics. Furthermore, “fuzz ball” sockets are relatively expensive due to costly manufacturing including the placement of individual wires into the through holes to form the various “fuzz ball” contacts. Additionally, the great force required to push the ball leads of a BGA package into contact with the “fuzz ball” socket creates wear on the BGA ball leads and increases the likelihood of distorting the ball leads.
Wadded wire contact performance is statistically based due to fabrication techniques. This means that the number of contact points and bulk resistance varies contact to contact which requires testing of every contact to verify performance and higher contact normal force. These contacts are also susceptible to physical handling damage. The spring rate of these contacts is relatively high with a low working range of compressions (i.e. about 3 mils).
Other contact assemblies use shear stamped LGA contacts. Such contacts typically have low compliance or high compression stiffness that requires a high nominal contact normal force to provide enough deflection to accommodate packaging tolerances. Stamped sheet contacts of a leaf spring design result in relatively long parallel contact structures with corresponding high electrical coupling which increases near and far end noise limiting signal integrity at high circuit (i.e. clock) speeds. Furthermore, it is desirable to achieve high contact stress and the connection interface, which results in a more reliable connection. Thus, there is a need in the art for a contact array that provides high contact interface stress with a low connection compression force, which in essence results in a low force.